Switchgears are used in electric power systems, with the purpose to control, protect and isolate electric equipment. In distribution nets, switchgears are located both on the high voltage side and the low voltage side of power transformers.
Switchgears used today for medium and/or high voltages are usually gas insulated switchgears (GIS), where the used insulating fluid is Sulphur hexafluoride (SF6). The use of SF6 allows for a compact design of the switchgear, since SF6 has good properties when it concerns dielectric performance and thermal performance, in particular related to preventing and dealing with unwanted electric discharges, e.g. arcs, and heat dissipation.
Generally, compact 3-phase switchgears used today for medium/high voltages AC power distribution systems comprise one to five modules inside a sealed encapsulation containing the insulation gas. Each module comprises typically a minimum of three switching assemblies, one for each phase. If the module is a circuit breaker module, each switching assembly would generally comprise a circuit breaker, a disconnector switch and an earthing switch that may be combined with the disconnector switch. If the module is a load break switch module, each switching assembly would generally comprise a load break switch that may be combined with an earthing switch, or combined with a disconnector switch with or without earthing switch.
As mentioned above, usually SF6 (sulfurhexafluoride) is used as insulation fluid/gas, which is very good for preventing and dealing with unwanted electric discharges, e.g. arcs. However, SF6 is a fluorinated gas that has a high global warming potential (GWP) and therefore alternative solutions are searched for. At the same time, it is desirable to keep the compactness of present switchgears with SF6, even if utilising other insulation fluids with lesser dielectric performance.
A switchgear and switching assembly is known from WO/2014/154292 in which a three-position switch is used for connection to the incoming electric bushing and earth, and a vacuum breaker is used for connection to the busbar, via the electrically conductive housing. In this document is discussed problems related to compactness of the switchgear and alternatives to SF6.
An example of a prior art switch is known from U.S. Pat. No. 7,767,917 that is fitted with field control rings.